+1 Best solution! Haven't seen this method yet. Just tested: it works perfectly. Interesting thing: SPListItem.UniqueId and SPListItem.File.UniqueId are the same guids.
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Andrey MarkeevApr 30 '11 at 13:42

@omlin - Thanks! GetFile and GetFolder They also have overloads with URLs, which can be very useful - it lets you access files ad folders without getting the library first.
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KobiApr 30 '11 at 13:48

Yep, actually I knew about these methods, but only about Url overloads :) And even if I have seen GUID overloads, how could one suppose, that listItem.UniqueId can be equal to listItem.File.UniqueId?! Very uncommon usage for property with such name, isn't it? :)
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Andrey MarkeevApr 30 '11 at 15:50

Thanks Kobi. I'll give it a shot as soon as I can!
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IunknownApr 30 '11 at 15:52

If you're using SharePoint 2010, look at the Document ID functionality (which attaches a unique ID to a list item and then uses Search to find the item). Depending upon your requirements, you might consider implementing "FindItemInSiteCollectionByUniqueId" using a similar approach to Document IDs, making the guid a Managed Property and conducting a search via code.